Field of the Invention
The invention relates generally to electric motors, and more particularly to systems and methods for construction of elongated electric motors in which the motor shaft can twist and skew the alignment of multiple rotor sections that are coupled to the shaft.
Related Art
Oil and natural gas are often produced by drilling wells into oil reservoirs and then pumping the oil and gas out of the reservoirs through the wells. If there is insufficient pressure in the well to force these fluids out of the well, it may be necessary to use an artificial lift system in order to extract the fluids from the reservoirs. A typical artificial lift system employs an electric submersible pump (ESP) which is positioned in a producing zone of the well to pump the fluids out of the well.
An ESP system includes a pump and a motor which is coupled to the pump and drives the pump to lift fluid out of the well. The ESP system may also include seals, gauge packages and other components. Because they are designed to fit within the borehole of a well, ESP systems are typically less than ten inches wide, but may be tens of meters long. The motor of an electric submersible pump system may produce hundreds of horsepower.
One type of motor that may be used in ESP systems is a permanent-magnet motor. In this type of motor, a long motor shaft extends through several rotor sections that are coupled to the shaft. Each of the rotor sections includes a set of permanent magnets that are typically positioned in multiple (e.g., four) axially aligned rows. The shaft and rotor sections are positioned within a stator. The stator has several coils or windings of wire positioned in a stator core. When the coils are energized, the windings generate magnetic fields that interact with the magnets of the rotor sections. The power provided to the stator windings is controlled to cause the magnetic fields of the stator to drive the rotor sections to rotate within the stator, thereby driving the rotation of the shaft. The rotating shaft is coupled to the ESP's pump and drives the pump to lift fluid out of the well.
There are several problems that may arise in the operation of permanent-magnet motors. Permanent-magnet motors are synchronous, so the rotor is always trying to stay at same rotational speed as the magnetic fields in the stator. The motor may therefore develop harmonic feedback that can be transmitted back to the motor's drive. This feedback may reduce the efficiency of the motor, and may even be harmful to the system. It would therefore be desirable to provide means to cancel out or reduce the harmonic feedback.
In addition to the problem of harmonic feedback, the motor may experience inefficiencies relating to the torsional rigidity (or lack of rigidity) of the motor shaft. Because the motor may be very long, the shaft may have a lower torsional rigidity at the bottom, as compared to the top. Since there is no slip in a permanent magnet motor, this leads to the lower-most rotor sections imparting less torque on the shaft than is provided by rotor sections mounted higher on the shaft. As a result, adding length, and more rotor sections, to a permanent magnet motor may provide diminishing returns with regards to torque output, power output, and efficiency. It would therefore be desirable to provide means to mitigate this problem.